Magnesia product and process of making the same



T. S. CURTIS Filed April '7, 1925 Nov. 6, 1928.

MAGNESIA PRODUCT AND PROCESS OF MAKING' THE SAME,

7%0/7705 JI CIJ/"7% BY ,Q l l ATTORNEY Patented I Nov. 6, 1928-.

, UNITED STATES .1,690,771 PATENT oFFicE.

THOMAS S. CURTIS, OF HUNTINGTON IPARK, CALIFORNIA, ASSIGNOR, BY' MESNE AS- SIGNMENTS, TO PACIFIC-SOUTHWEST TRUST 66 SAVINGS BANK, TRUSTEE, OF LOS ANGELES, CALIFORNIA, A CORPORATION OF CALIFORNIA.

VMAGNIISIA PRODUCT AND PROCESS OF MAKING THE SAME.

Application ledApril 7, 1925. Serial No. 21,348'.

This invention relates to the/.production of magnesia in the formi of periclase and the chief objectof the invention is to produce such form of magnesia of an especially high degree of purit-y land having novel and advantageous properties resulting from s uch puity and from the method by vvhlch 1t 1s ma e.

The conversion of magnesia to the crystalline form of periclase requires very high temperature and heretofore such temperature has been obtained for this purpose 1n commercial practice only in the electric furnace. The production of periclase in the electric furnace however` is attended with vmany disadvantages, for example in regard to the purity of the product since perlclase produced in this manner invariably contains certain impurities, the introduction of which into the product can not be avoided in an electric furnace and the presence of which seriously impairs the value of the product for certain uses. The impurities thus introduced in the electric furnace treatment and Which are disadvant-ageous include carbon from the electrodes, iron from the metal Walls of the furnace, and compounds formed between such carbon and iron and other impurities naturally occurring in the mag- -nesia from which t-he periclase is made. An example of this latter class of impurities is calcium carbide, formed by reaction between traces of lime existing in the magnesia and the carbon of the electrodes.

These impurities render the product hygroscopic, causing it to absorb or take up considerable quantities of Water so that theY powdered material cakes or forms lumps and hence becomes unsuited for certain uses. rlihese impurities also increase the electrical conductivity of.. the material and hence seriously interfere with its use as an electrical insulator'. Furthermore the impurities not only increase. the electrical conductivity directly by their presence, but the increased amount of water which their presence causes to be absorbed by the material still further increases the electricalconductivity.

The periclase produced according to my invention and having the properties hereinafter stated is free from these objectionable impurities and is therefore substantially non-hygroscopic and has a very low electrical conductivity. When powdered it Will not cake together even in wet weather and inv such form it is ve well adapted for use as an electrical insu ator.

A further object of the invention is to produce riclase in a more finely crystalline con ition than it has heretofore been produced, thereby facilitating the reduction or grinding of the product to the desired degree of fineness for use.

A further object of the invention is to produce periclase by heating magnesia to a temperature below the fusion point thereof, so as to obtain the above-mentioned advantages of finer crystals, etc., and to also obtain a product which is of a distinctly granular nature, as distinguished from the dense, stony nature of the periclase heretofore produced.

The usual method of forming periclase, and in fact the method which has been used 'almost exclusively in the commercial production thereof, consists, asstated above, in heating a charge of magnesia tol the ushion point; and then allowing the fused mass to cool and crystallize. By this method of procedure, relatively large crystals of periclase are obtained, and such crystals are all held together in a dense, almost vitreous mass which is very diiiicult to grind to proper size for use. urthermore, by this method of product-ion the small amounts of sili- .ceous impurities Which are present in the raw material, do not appear to form glass but apparently go into solid solution in the magnesium oxide due to the yfushion thereof.

According to my method, Which may'be termed the converter method as distinguishedfrom the above-described electric furnace method the magnesia is converted to the form of periclase Without heating to the fusion point, and due to this fact the crystals which are produced are very much smaller and more uniform in size and are held together relatively loosely, with a considerable percentage of voids so that the product is of a distinctly granular character and is relatively easily ground or reduced to the desired size. Furthermore the siliceous impurities react with a portion of the magnesia to form a small amount. of glass, probably chiefly magnesium silicate, which glass examination 'of the product shows to exist in the form of a thin coating' on each periclase crystal or at least on a coated particles and thus prevents caking of 'substantial proportion of `such crystals.

This glass coating on the individual grains, I believe to be of material advantage for the reason that it prevents or materially inhibits the absorption of moisture by the the powdered material on standing and also maintains the electrical conductivity of the material at a very low figure.

The accompanying drawings illustrate an apparatus suitable for carrying out my heating process for the production of periclase and referring thereto:

Fig. 1 is a side elevation of such apparatus.

Fig. 2 is a vertical section through the converter, on line 2 2 in Fi 1.

. Fig. 3 is a horizontal sectlon on line 3-3 in Fig. 2. j

The converter or vessel in which the material is heated comprises preferably a vertical, cylindrical shell or casing 1 whose side and bottom walls are lined with suitable refractory 'or heat resistant material indicated at 2. The converter is open at its upper end to permit charging and emptying and is provided with a removable cover 3 constructed of or lined with refractory material for closing the upper end thereof during certain stages `of the converter operation as hereinafter described, said cover having a suitable vent-or opening 3'. Said converter is mounted lto tilt in a vertical plane on horizontal axes or trunnions 4, suitable means indicated at 5 being provided for effecting such tilting operation. Supply means 6 are provided for mixing and supplying gas and air to the converter, said supply means having detachable connection 7 adapted to be disconnected so as to permit tilting of the converter. Said supply means 6 is connected by suitable branch pipes 8 to tuyres 9 extending through the.

bottom of the converter andfadapted to supply or introduce gas and air into the converter. Suitable means are provided for producing a supply of air under pressure, such means comprising, for example, an air compresser or blower 12 connected by pipe 13 to the mixing and supply means 6. Gas supply means indicated at 14 are also provided for introducing into said supply means 6 the supply of the fuel gas. Said ai r and gas supply means 13--and 14 are provided with suitable valve means indicated at 15 and 16 respectively. Suitable means such as friction plugs or caps 18 may be provided at suitable points in the air su ply pipe and in the fuel and air supp y means 6 for preventing dangerous explosions, such plugs or caps being fitted comparatively loosely in place and adapted to be blown out in case of explosion, thus preventing damage to any parts of the apparatus.

My process of manufacture may be carried ous as follows: The raw material required is magnesite ore, preferably with a high magnesia content and as free as possible from impurities. In some cases pure magnesia may be used as raw material and in such cases no primary purification or calcining is necessary. In general however the ore as received is partly or largely in the form of magnesium carbonate or magnesite and contains more or less iron and other impurities. I therefore prefer to subject the ore to a primary calciningl and purifying or grading operation.

This primary operation may consist in first washing the ore to remove therefrom any dirt and foreign matter adhering to the surface of the lumps or fragments of ore, then calcining to a bright red or orange heat to remove carbon dioxide and convert the maglnesite to magnesia. or magnesium oxide. `his calcination serves a further purpose of converting particles or fragments of impurities, such as iron or iron oxide, to a form in which they may be more readily detected due to the distinctive color of such materials, the magnesia itself being of a white color. After the calcining operation I therefore refer to subject the material to a rough and sorting operation in which all slagged material as well as all black, brown or other colored pieces are removed. The white material passing through this sorting operation is then crushed to a suitable state of division, for example to approximately twenty mesh. lThe crushed .material is then subjected to a further manual sorting operation in which all specked or discolored articles are removed. It will be understoo that the above urifying or sorting operations are solely or the purpose of removing naturally occurring or other impurities from the material and do not constitute an essential part of my invention, except in that they provide a I neans for obtaining a magnesia of a sufiiciently high purity from which tlie desired finished product maybe produced.

The crushed magnesia free from impurities is then agfrregated into lumps or fragments of suitabe size for charging into the converter. For this purpose I prefer to puddle the pulverized material with water and form it into cakes or blocks of convenient size, for example approximately 50 pounds each. These cakes are then dried in any suitable manner for 4example by allowing them to stand in the air or by passin them on cars through a suitable dryer o anywell-known type. In some cases the heat of hydration developed by the absorption of water by the magnesia may be sufcient to dry the cakes without application of further heat. The dried cakes are then broken or crushed to suitable size for -example to fragments averaging from 2 to 4 inches across, this crushing being done either Y by hand or in any'suitable Crusher.

The fragments or aggregates so obtained arethen charged into the converter until it is substantially `full, the only 'precaution necessary during this charging operation being to .insert rods or bars in the openings of the tuyres at the bottom of the' converter at the start of the charging operation so as to prevent covering of such openings by any of the fragments of the material. The bars used for this purposeare removed as soon as the first portion of the charge isv placed.

The converter operation may be divided into two principal stages. The irst, which may be called the gasor ignition stage. ccnsists in burning an explosive mixture of natural gas or other combustible carbon-bearing gas and air in the converter which in many respects is similar to a typical Bessemer converter as used in the steel industry. In starting the converter, -gas is admitted through-pipes 14L and 6,without air, and ignited at the top of the converter. As soon as free combustion is established the air valve 15 is opened until an ex losive mixture is obtained which is imme iately manifested by a mild explosionv which extinguishes the free burning gas Hame at the top of the converter and. promotes the-combustion without Visible flame down inside the charge within the converter.` lThe proper tions of air and gas necessaryv for. this combustion are readily calculated if'the composition and B. t. u. -value of the gas are known. In the case of a typical Southern California natural gas having a value of approximately throughout the mass as hereinafter -de- 11750 B t. u. the proportions of 'gasto air for erfect combustion are as 1 to 10.7. In orer to ensure suiiicient deposition of carbon "scribed, however, I prefer in 'general to use al mixture containing a slight excess of gas. For this purpose the ratio of gas to air may be maintained for example at 1 to 10. This ldesired mixture may be controlled for `ex- Y ample by meansV of. low pressure direct readpressure per squareinch. Air is supplied by ing manometers 19 which read in ounces of -a positive pressure blower or other means 12 operating preferably' at from 11/2to 2 pounds to thesqua-re inch. Gas isdelivered to the furnace at a suitable pressure for example approximately 6 pounds per square -inch ahead of thecontrol valve. The actual gas pressure registering onthe gauge be- ,"yond the controll valve is maintained for example at s ix ounces above the air pressure.

This relation is, ofjcourse, established by *61" `the size of the air supply pipe and the orices throughwhich the gas passes into' the mixing chamber or supplypipe 6. The relation ofthese openings is purposely de lsigned to maintain constantly. a higher gas pressure than air pressure at the mixing Aexplosions in the gas line.

point to prevent air passing bac-k into the gas pipe line, and thus prevent danger of The' friction caps 18 above described serve to prevent serious damage in case of explosion within the ini.'- ing ciamber 6. Such explosion hazards have however virtually been eliminated by reducing the size of the tuyres 9 and in creasing the velocity of the explosive mixture until the velociv was found to be greater than the .rate of flame propagation.

Once the combustion has been started the operation of the converter is characterized by extreme simplicity, the fact that coinbustion -is taking place being manifested solely by a roaring sound from within the converter. Usually for the first two to three hours of the operation no flame whatever is visible in the converter, and usually fragments of pure magnesia can be added to that originally placed in the converter, due to the settling of the charge brought about4 by the partial fusion or softening thereof and the reaction occurring therein. During all the above stages of the operation of the converter, the removable cover 3 is left o, leaving the-upper end of the converter open. Usually after sixto fourteen hours of operation the portable cover 3 is placed on the converter to complete the burning of the charge at the extreme top of the vessel. The balance of the gas period, which in total comprises from ten -to twenty hours or longer, is then merely an operation of quiet` combustion. v

It will be seen that during the above described gas period an explosive mixture of gas `and air is supplied to the bottom of the bed of fragmentary material in the converter and the combustion of this mixture takes lace at the surfaces of such material. It 1s a Well-known fact that the type of combustion thus produced which is known as 4surface combustion produces much higher .temperatures than can be obtained when the gas and air mixture is burned in an open `fiame. The entire mass of fragmentary material is thus raised to a high temperature, vthis temperature being at or neai" that at lim III

the lumps during the combustion due to decomposition and incomplete combustion of the gas.4 I prefer therefore and in general find it necessary for the production of a satisfactory product to follow the above described gas period by an air or blast period which constitutes the second phase of the converter operation.

For this purpose at the conclusion of the gas period the gas is turned off and air alone is admittedfro-m the blower for a .suitable length of time, for example from two to eight hours or longer, the cover 3 being left on during this stage of the operation. The first part of this blast period is characterized by a distinct rise in temperature and the mass of -fragmentary material remains redhot during substantially all of the blast period. This indicates the evolution of a considerable amount of heat during this period and this I believe to be due to the oxidation of the carbon deposited within the lumps of the material. The heat thus developed is not only very intense but is developed in contact with the material and j throughout the entire body thereof and results in a very complete conversion of the magnesia to the form of periclase.

At Athe end of a suitable period which is determined by experience and by the appearance and behavior of the material in `the converter the air blast is discontinued.

crystals of minute or microscopic size, may

be considered to comprise substantially pure perlclase I prefer to subject the same to\a further sorting and purifying operation ac companied by a crushing andgrinding ofthe material to any desiredstate of division. These subsequent operations form no essential part of my invention except inasmuch as they lead to the ultimate production of the converter product in a very pure stateand in a marketable form.

For this purpose the material discharged from the converter may be allowed to cool sufficiently to permit handling thereof and is then vs orted by hand. Any fragments which are apparently not entirely crystallized or converted to the form of periclase are returned'to be mixed with. the raw material. Such fragments only occur in general at the extreme topv of the converter and 'at the bottom immediately over the tuyre openings, so that this classification may be easil ,mada v'Ihe remainder of the product 1s then crushed and screened to produce a product of approximately l@ inch mesh and is then subjected to a magnetic separating operation to remove particles of iron or iron oxide introduced mechanically during the crushing operation. The iron-free material is then subjected to a final grinding operation for example in a pebble mill in which it is ground to any desired grain size for example 60 mesh.

It may be mentioned in this connection that the finel crystalline condition of the periclase pro uced by the converter operation above described is of very great -practical advantage in connection with the grinding of such product to the desired degree of fineness for use. The peric'lase heretofore produced by fusion of magnesia in an electric furnace has always been in the form of relatively coarse crystals bonded together in a dense stony mass so that reduction ofthe same to the necessary state of division re- Bil quired not only the breaking up of the hard y stony fragments of material but also the breaking up of the individual crystals. A

relatively hard grinding was consequentlyy required, accompanied by the production of a large amount of fines or powder of less than the desired size-and a repeated or continual separation of the material of the desired ineness or less from the larger material during grinding, thus resulting not only in a non-uniform product but also in a relar De) tivel long time and high cost of grinding.

In t e case of periclase produced by my process on the other hand the crystals are so minute that it is only'necessary to grind .sufficiently to substantially separate the crystals, or groups of crystals from one another, without requiring appreciable breaking up of individual crystals. Furthermorel the crystals are not bonded together in a dense stony mass but are more loosely associated in fragments of a granular nature which are much more easily reduced to the desired size. This grinding is very readily accomplished and the roportion of fines produced is much smal er than-in the case of an electric-furnaceproduct and little or no removal of the fine material during grinding is necessary thus resulting in decreased time and cost of grinding as well as greater uniformity in size of the product.

Furthermore the4 production of .periclase withoutfusion results in the formation of a vsmall amount of magnesium silicate 'glass and in the uniform distribution of `such glass particles or crystals, whereas in thek coarsely crystalline fused product of the electric furlili lau

nace, the major portion of the silica apapproximately .2 mm. and only about .1%of parently goes into solid solution inthe mag- .l0 mm. or less. Microscopic examination nesia, while any glass which is formed isV also shows the presence of a coating of sililumped or unevenly distributed in the prod`v ceous glass on the major portion of crystals uct. Since the glass which is vformed conof periclase formed by the converter method tains substantially all of "the impurities and a substantially total absence of such which are present it will be seen that in .the coating of converter product such impurities' while. product. present in very minute amounts are uni' 'Phe heating process -above referred to by formly distributed over they surface oftlie which the magnesia is converted into the glass in the electric furnace crystals, While in the electric'furnace prodformof periclase consists, as stated, of two uct the distribution ot such impurities is stages; iirSt, the combustion, indirect conmuch less uniform; Furthermore the coattact with the surfaces of the lumps or fraging of glass on the individual particlesis ments of magnesia, of an explosive mixture substantially imperviousto moisture and I of gas and air whereby the entire mass is believe this `coating of glass to be an im- Y raised to a high temperature andnely diportant factor in preventin or inhibiting vided 'carbon is deposited throughoutthe the absorption of moisture by the periclase lumps or fragments; andv second, passing air produced in accordance with my invention. through and in contact with the surfaces of The low absorption of moisture isv of adthe resulting hot body of materiaLcauSng vantage for the reasons. above stated. Y the deposited carbon to be burned throughl Theresulting productis a dry white powthe materiahwhich in .turn causes a further der which is substantially non-hygroscopic raise in temperature landa substantially com- A uctivity. plete conversion ofthe mav'nesia to the crysand has a very low electrical con t is apparent that Even after standing for a long period of talline form of periclase.

time in wet weather it does. not cake toin the first stage the combustion takes place `gether like the periclase ordinarily produced not .only at the surface of the fragments 1n theelectrical furnace, such caking being but to some extent at least within the voids due to absorption of water from the air. No inside the fragments themselves, for the dep' diiiiculty is experienced in' practicel in mainosition of finely divided carbon takes place taining the mosture content of the product practically homogenously throughoutl such well below one tenth of one 'per cent, and in fragments. This indicates that the fragfact vthe mosture content is usually not over ments or lumps of which the charge is 'made two of three .hundredthsof one -per cent. up, are iinelyporous and therefore gas'- The low electrical'conductivity of the' mate-x permeable. n the same way the oxidation rial renders it very useful for electrical inof this carbonduring the air blast period sulation work, the powder being for this purtakes place throughout and in intimate conpose packed or tamped inplace around the tact with substantially the entire mass of electrical conducting members to be inmaterial; I have found that a higherrtemsulated. A particularly advantageous apperature can Vbe attained by rst burning a plication of this material for electrical inmixture slightly deficient lin oxygen so as to sulation is inthe insulation of heating coils deposit carbon in the charge and then burnf in 5 'which the as above descr ed is tightly packed around by simple combustionv of a perfect mixture the heating coil'in a suitable container. The ot'gas and air in and around the charge.

powdered periclase produced vingl out this carbon with ablast of air than' 1 :tact that this material in addition. to being Whether thedeposition of carbon and its an electrical insulator is a comparatively subse uent removal b air blast is of benefit lgood heat conductor and is unaffected. b only ecause of the eating to a relativelyhigh temperature i\p also of further advantage in this connectained has somecatalytic effect mpromottion. Y Y ing the conversion of the .magnes1a 'tofthe Macroscopic and microscopic' examination form of periclase, 4I can not. state with ceri h temperature thus-fl I roduced or' whether t e reaction thusA obne ofsamples of periclase produced by the'elec- -tainty, but tests have shown a much more tric furnace method and by the converter ycomplete conversion vwhen-the rocess'is carmethod above described shows that in the ried out as above descr1bed t an when the -converter product the average grain size 1sl magnesia is heated by any other means on the order of 1/100 as great as inthe elecknown to me, with the possible exceptionp tric urnaceproduct. The grain size of the 'of fusionjin the electric furnaces. The proconverter product has been found to vary duction ofpericlase in the'electric furnace on between 1010 mm. and .042 mm. with an averthe other hand issubject to the disadvanage sizeof about .028 mm. and substantially tages above set forth namely the contamina- 80% between .017 mm. and .035 mm. In the tion of the roduct by the impuritiesVA inelectric furnace product on the other hand evitably resu ting fromv the electric furnace Aapproximately 99% of the crystals are 1 mm. operation and the relatively coarsely 'crystal'- in diameter or larger with only about 1% of line nature andconsequent diilicult grinding `ously comprise re ractory bricks or blocks of magnesium-aluminate. It is to be noted however that any possible contamination which might result from the converter lining even with the use of the purest materials therein would be confined to a comparatively small proportion of the entire converter char e and the portion thus contaminated coul ...easily be separated inthe subsequent sorting operation and even if not removed would have no appreciable effect on the composition of the entire mass.-

I have not been able to determine with accuracy the exact temperature4 attained in the converter operation but such measurements asv have been made indicate that this temperature is a proximately ceramic cone 40 or 3506 F. ido not wish to be limited therefore to any exact temperature but may employ any temperature which may be obtained by the above-described method and which is suiicientunder the conditions `set forth to cause substantially complete conversion of the magnesia to the form of periclase. It will be noted' however that the above temperature is. well below the melting point of magnesia which is about 5070 F. and I believe that the production of minute crystals of silica as wella's the formation of the extremely thin glass coating on the' individual crystals is due to the fact that the Amagnesia is converted to the form of periclase without fusion. The production of periclase without fusion also results in the ormation of agranular product which is relatively easily disintegrated as distin.

l,guished from the dense, stony, diiiicultly ground product produced by fusion in the electric furnace. Furthermore, by this method, the individual fragments or aggregates originally charged into the converter .retain substantially their original shape and arenot substantially fused together to form a solid mass. The temperature to which the i .fragmentary body of magnesia is heated may therefore be defined as sufficient toy cause substantially completer conversion of the magnesia to crystalline form but insufficient .to cause fusion thereof or to cause the fragments of the charge to substantially fuse together.

I claim.:

1. A lmagnesia product consisting of magnesia which has been heated suiiciently to convert the same substantially wholly to periclase without fusion thereof.

2. A magnesia product consisting substantially of fragments composed of magnesia crystals produced by heating fragments of magnesia to a temperature sufficient to convert the magnesia to crystalline form but insufficient to substantially fuse such fragments together. f

3. The process of making a magnesia product which comprises heating a body of perature and cause deposition of carbon therein, and then passin air through said body of magnesia to oxi ize such deposited carbon and cause the magnesia to be converted to the form of periclase due to the heat developed by such oxidation.

5. The process of making periclase which comprises heating a` fragmentary body of magnesia to a substantially uniform temperature which is suiiicient to convert the magnesia substantially to the form of periclase but insuiiicient to substantially fuse together the fragments of such body.' A

6. A magnesia product consisting substantially of ericlase cr stals, a substantial proportion o the indivi ual crystals having a thin coating of glass thereon.

7. A` magnesia product consisting of a granular non-fused body of crystals.

8. A. magnesia product as set' forth in claim 7, said crystals having an average diameter of less than'one tenth of one milli! meter and a substantial proportion of said crystals having a coating of glass surrounding the same.

9. The processof making periclase which' comprises'the steps of impregnation of a mass of highly heated magnesia with carbon and then oxidizing the deposited free car. bon by air blast.

10. The method of producing a ma nesial product in the form of periclase whic con-- sists in heating a body of magnesia to a high temperature and while maintaining such temperature impregnating the mass with carbon and then rapidly oxidizing the carbon.

11. The method of producing periclase which consists in heating the magnesia by combustion of fuel proportioned to have an excess of carbon for a suiiicient period of time to effect a deposit of carbon substantially throughout the mass of magnesia and then producing an increase in temperature by rapid oxidation of the carbon.

12. The method of producing periclase Which consists in subjecting magnesia to heat by combustion of air and fuel gas in Contact with thesame for a substantial period of time to produce high temperature in the mass and effecting a deposit of free carbon in the mass by the gas and then discontinuing the supply of gas and blasting with air to rapidly act upon the uncornbusted carbon for continuation of the heat ing process at higher temperature until conversion to crystalline form.

In testimony whereof I have hereunto sub- 20 scribed my name this 28 day of March, 1925.

THOMAS S. CURTIS. 

